System and method for providing auxiliary power by regeneration power management in mobile mining equipment

ABSTRACT

A method for operating an ultracapacitor system used in a mining excavator powered by an electrical power source. The method includes the step of detecting whether a line power from the electrical power source is present. Next, a voltage level of the ultracapacitor system is measured if the line power is not present. The measured voltage level is then compared with a minimum voltage level for the ultracapacitor system. If the measured voltage level is more than the minimum voltage level, auxiliary power is supplied from the ultracapacitor system to operate mining excavator systems. The auxiliary power may be used to power electronic systems and components such as computers, displays, control systems, gas insulated switchgear and lighting systems.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/466,722 filed on May 15, 2009 entitled LIMITING PEAKELECTRICAL POWER DRAWN BY MINING EXCAVATORS which is incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to electrical power systems formobile mining equipment such as mining excavators and off road haultrucks, and more particularly, to an electrical power system which usesenergy stored in an energy storage system as auxiliary power source forvarious mobile mining equipment electric and electronic systems.

BACKGROUND OF THE INVENTION

Mobile mining equipment such as mining excavators and off road haultrucks is used during mining operations. Mining excavators such aselectric shovels and draglines draw a significant amount of electricalpower during mining operations. The power drawn by a mining excavatorpresents a cyclic load to an electrical power distribution network. Thecyclic load may include at least one motoring interval (wherein power isdrawn from a power source) and one regenerating interval (wherein poweris returned to the power source) Electrical power generated by themining excavator during the regeneration interval is captured and usedto charge an on-board electrical energy storage system.

Mining haul trucks are typically equipped with an electrical drivesystem that includes electric motors which drive the truck wheels. Thehaul truck draws electrical power from an electrical generator poweredby the truck engine or an overhead trolley line while propelling. Duringdownhill travel, the electric motor brakes and this braking power iscaptured and used to charge an on-board electrical energy storagesystem.

The system or device used for energy storage is an important area ofconcern in mobile mining applications. As an example, mining excavatorsor haul trucks frequently operate in remote locations with extremeclimatic conditions where temperatures may go as low as 20° C. belowzero. This puts a severe strain on traditional energy storage systemsthat utilize batteries and associated systems which are used for backuppower for various electronic systems and components in the miningexcavator such as computers, displays, control systems, protectionsystems and others.

As an example, the control power and backup power for gas insulatedswitchgear (GIS) used in a mining excavator is typically provided bylead-acid DC batteries. Such batteries are sufficient for normalconditions but have substantial disadvantages when used in severe coldweather conditions since the acid in the batteries is susceptible tofreezing. In order to avoid this, the batteries are typically stored ina heated compartment. However, if a prolonged power outage occurs thetemperature of the compartment is no longer controlled which leads tofreezing and ultimately battery failure.

As previously described, the regenerated power produced by an excavatoror a haul truck may be stored as energy in an energy storage system.This stored energy could be used to reduce the peak power demanded bythe electric motors in an excavator or haul truck. Referring to FIG. 11,a typical duty cycle for an electric shovel is shown as power plot 1200.As can be seen, the power profile for the electric shovel indicates acycle time of approximately 32 seconds. This indicates that the energystorage system should have the capability to charge and discharge every32 seconds. However, subjecting a conventional battery to such frequentcharge/discharge cycles undesirably reduces battery life. Anotherdisadvantage of using batteries is that they require regularmaintenance. In addition to reducing peak power demand, the storedenergy could be used as an auxiliary power source for various systemcritical components like protection system, control system etc.

Alternatively, capacitors have been considered for use in an energystorage system instead of batteries. However, the power requirements ofa mining excavator would require the use of very large capacitors. Thisis undesirable since shovels are mobile machines and size and weight areimportant parameters in shovel design.

BRIEF SUMMARY OF THE INVENTION

A method is disclosed for utilizing the energy storage capabilities ofan ultracapacitor system used in a mining excavator powered by anelectrical power source. The ultracapacitor system is charged to acertain maximum voltage to store energy. The ultracapacitor isdischarged to a certain minimum voltage when providing power. The methodincludes the step of detecting whether a line power from the electricalpower source is present. Next, a voltage level of the ultracapacitorsystem is measured if the line power is not present. The measuredvoltage level is then compared with a minimum voltage level for theultracapacitor system. If the measured voltage level is more than theminimum voltage level, auxiliary power is supplied from theultracapacitor system to operate mining excavator systems.

These and other advantages of the invention will be apparent to those ofordinary skill in the art by reference to the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a high-level schematic of an electric shovel;

FIG. 2A shows a high-level schematic of a cyclic load drawing electricalpower from an electrical power source;

FIG. 2B shows a plot of the power demand of a cyclic load with noregeneration;

FIG. 2C shows a plot of the output power drawn from the electrical powersource shown in FIG. 2A corresponding to the plot of power demand shownin FIG. 2B;

FIG. 2D shows a plot of the power demand of a cyclic load withregeneration;

FIG. 2E shows a plot of the output power drawn from the electrical powersource in FIG. 2A corresponding to the plot of power demand shown inFIG. 2D;

FIG. 3A shows a high-level schematic of a cyclic load drawing electricalpower from an electrical power source and an electrical energy storageunit;

FIG. 3B shows a plot of the power demand of a cyclic load withregeneration;

FIG. 3C shows a plot of the output power drawn from the electrical powersource shown in FIG. 3A corresponding to the plot of power demand shownin FIG. 3B;

FIG. 3D shows a plot of the power demand of a cyclic load with noregeneration;

FIG. 3E shows a plot of the output power drawn from the electrical powersource in FIG. 3A corresponding to the plot of power demand shown inFIG. 3D;

FIG. 4 shows a single-line diagram of an electric shovel control system;

FIG. 5 shows a schematic of an electrical power system with anintegrated ultracapacitor bank;

FIG. 6 shows a plot of the power demand of an electric shovel;

FIG. 7 shows a plot of the output power drawn from an electrical powersource when the output power is constrained between an upper limit and alower limit;

FIG. 8 shows a plot of the output power drawn from an ultracapacitorbank;

FIG. 9 shows a plot of the stored electrical energy in an ultracapacitorbank; and

FIG. 10 shows a flowchart of steps for limiting the input power betweenan upper limit and a lower limit.

FIG. 11 depicts a duty cycle for an electric shovel.

FIG. 12 depicts a method for operating an ultracapacitor bank.

FIG. 13 depicts power plots in accordance with the present invention.

FIG. 14 depicts energy requirements for gas insulated switchgear (GIS).

FIG. 15 depicts a single-line diagram of a haul truck power system.

DETAILED DESCRIPTION

Mobile mining equipment such as mining excavators and off road haultrucks is used during mining operations. Mining excavators includeelectric shovels and draglines. FIG. 1 shows a schematic of an electricshovel 100 to illustrate a mining excavator that consumes significantelectrical power. The major components are crawler 102, deck 104, boom106, hoist 108, handle 110, and dipper 112. Electric motors enablevarious motions to operate the electric shovel 100. Motion 131 propel(forward/reverse directions) refers to travel of the entire electricshovel 100 with respect to the ground. Motion 133 swing (away/returndirections) refers to rotation of deck 104 with respect to crawler 102.Motion 135 crowd (crowd/retract directions) refers to positioning ofdipper 112 with respect to boom 106. Motion 137 hoist (hoist/lowerdirections) refers to positioning dipper 112 up and down with respect tothe ground. Multiple electric motors may be used to provide each motion.

An electric shovel typically performs a series of repetitive operations.For example, it may propel forward near a bank, swing the dipper intoposition, crowd the dipper into the bank, hoist the dipper to scoop outmaterial, retract the dipper, propel in reverse to clear the bank,propel forward to a dump site, swing the dipper into position, lower thedipper, and dump the load. It then returns to the bank and repeats theoperation. Motors, then, often accelerate in one direction, brake, andaccelerate in the opposite direction. The mechanical load on a motor ishighly variable. As one example, consider a motor hoisting a dipper fullof heavy material, dumping the material, and lowering an empty bucket.

Mining haul trucks are typically equipped with electrical drive motors.FIG. 15 shows a single-line diagram of a haul truck power systemincluding an overhead trolley line power system. The haul trucktypically includes two drive wheels. Each wheel is driven by a 3-phasealternating-current (AC) wheel motor (M). The wheel motors arereferenced as wheel motor 1500 and wheel motor 1510. Electrical power issupplied by a 3-phase AC generator (G) 206 which is driven by a dieselengine 1520. It is noted that other types of mechanical engines may beused although it is understood that diesel engines are typically used inmining operations. Coupling 1530 connects diesel engine 1520 togenerator 1540. Diesel engine 1520 and generator 1540 are mounted on thehaul truck. The AC output of generator 1540 is fed into rectifiers 1550.The direct current (DC) output of rectifiers 1550 is fed into a set ofinverters 1560 and 1570. Inverters 1560 supply 3-phase AC power to wheelmotor 1500. Similarly, inverters 1570 supply 3-phase AC power to wheelmotor 1510.

Alternatively, the inputs of inverters 1560 and inverters 1570 may alsobe connected to DC power supplied by electric substation 1580 viatrolley line 1590 and trolley line 1600. Electrical connection of thehaul truck to trolley line 1590 and trolley line 1600 is implemented viapantograph arm 1610 and pantograph arm 1620, respectively. Throw switch1630 connects/disconnects the inputs of inverters 1560 and inverters1570 to trolley line 1590 and trolley line 1600. There is also anauxiliary breaker 1640. When the haul truck is connected to trolley line1590 and trolley line 1600 on an uphill grade, a large power demand isplaced on the electric substation 1580, resulting in a drop in DC linkvoltage and increased current flow through trolley line 1590 and trolleyline 1600.

The present invention will be described under conditions wherein theentire power requirements for wheel motor 1500 and wheel motor 1510 aresupplied by diesel engine 1520 and not by electric substation 1580.

Braking of a haul truck is a critical part of the transport operation.Heavy loads, steep grades, and fast movements result in large quantitiesof mechanical energy which needs to be managed. Under normal operation,an electrical motor converts electrical energy into mechanical energy.An electrical motor may also be operated in reverse as a generator toconvert mechanical energy into electrical energy, which is fed intoinverters. In typical dynamic braking systems, braking choppers,connected to the inverters, channel the power into a power resistor gridthat continuously dissipates the energy until the truck reachesstandstill. Braking is smooth, similar to the braking operation in acar, but without mechanical brake wear. Referring to FIG. 15, forexample, chopper 1650 and power resistor grid 1660 provide the brakingaction for wheel motor 1500. Similarly, chopper 1670 and power resistorgrid 1680 provide the braking action for wheel motor 1510. In typicaldynamic braking systems, therefore, the regenerated electrical energy(also referred to as retard energy) is converted into waste heat. Inembodiments, as described in detail below, however, the regeneratedelectrical energy is captured and recycled.

The procedure of utilizing the stored energy as an auxiliary powersource will be explained with respect to the operation in electricshovels although it is understood that the present invention is alsoapplicable to mining haul trucks.

From an electrical power perspective, an electric shovel presents acyclic load to an electrical power source. As a function of operatingtime, the electrical power drawn by the electric shovel variescyclically. The variation in power may be significant as the averagepower drawn by these machines may be about 55% of their peak powerdemand.

Under normal operation, an electric motor converts electrical energyinto mechanical energy. An electric motor may also be operated inreverse as a generator to convert mechanical energy into electricalenergy. Under normal operation, an electric motor draws (consumes)electrical power from an electrical power source. When an electric motorunder motion is stopped, the residual mechanical energy may be convertedto electrical energy. Herein, a time interval during which an electricalload is drawing electrical energy is referred to as a motoring interval;and a time interval during which an electrical load is generatingelectrical energy is referred to herein as a regeneration interval.

In FIG. 2A, electrical power source 202 feeds total user load 204. Inthis example, total user load 204 comprises application load 206.Controller 208 controls the electrical power transferred betweenelectrical power source 202 and application load 206. Electrical powerP₁ 221 represents the output electrical power from electrical powersource 202. Electrical power P₂ 223 represents the input electricalpower drawn by application load 206, which, in this example, is a cyclicload. The input electrical power required to operate a load is alsoreferred to as the electrical power demand of the load.

FIG. 2B shows plot 230 of the electrical power demand P₂ 223 (verticalaxis) of application load 206 as a function of time t (horizontal axis).In this example, P₂ ranges from zero to positive values. When theelectrical power demand is positive, application load 206 is drawingelectrical power. Plot 230 is a generic plot used for illustration. Theactual power demand is dependent on the specific equipment and operatingconditions. For simplicity, many portions of plot 230 are shown asstraight line segments. In general, the shape is arbitrary (for example,curvilinear).

In FIG. 2B, cycle 231-cycle 247 are examples of cycles. Note that thecycles are not necessarily strictly periodic. The functional dependenceof power vs. time, the amplitude, and the duration of each cycle mayvary. In addition to up/down variations, a cycle may include othergeometric features, such as a plateau (constant power) in cycle 237 anda cusp in cycle 241.

FIG. 2C shows the corresponding plot 240 of the output electrical powerP₁ 221 (vertical axis) from electrical power source 202. When the outputelectrical power is positive, electrical power is drawn from electricalpower source 202. In the example shown, the output electrical power P₁221 (plot 240 in FIG. 20) is equal to the electrical power demand P₂ 223(plot 230 in FIG. 2B).

FIG. 2D shows a different example, plot 250, of the electrical powerdemand P₂ 223 (vertical axis) of application load 206 as a function oftime t (horizontal axis). Note that the power ranges from positive tonegative values. When the electrical power demand is positive (motoringregion), application load 206 is drawing electrical power. When theelectrical power demand is negative (regeneration region), applicationload 206 is generating electrical power. As shown in plot 250,application load 206 generates electrical power during time interval 251(t₁≦t≦t₂), time interval 253 (t₃≦t≦t₄), and time interval 255 (t₅≦t≦t₆).

FIG. 2E shows the corresponding plot 260 of the output electrical powerP₁ 221 (vertical axis) from electrical power source 202. When the outputelectrical power is positive, electrical power is drawn from electricalpower source 202. When the output electrical power is negative,electrical power is fed back into electrical power source 202. In theexample shown, in the motoring region, the output electrical power P₁221 (plot 260 in FIG. 2E) is equal to the electrical power demand P₂ 223(plot 250 in FIG. 20). In the regeneration region (time interval 251,time interval 253, and time interval 255), the output electrical powerP₁ 221 is zero. In this example, the electrical power generated in theregeneration region is fed into a resistor (not shown) and converted towaste heat. The electrical power generated in the regeneration regionmay also be fed back to electrical power source 202. The outputelectrical power P₁ 221 would then be negative during time interval 251,time interval 253, and time interval 255.

FIG. 3A shows a schematic of an electrical power system, according to anembodiment of the invention, that recaptures the electrical powergenerated in the regeneration region. Electrical power source 302 feedstotal user load 304. In this example, total user load 304 comprisesapplication load 306 and electrical energy storage unit 310. Controller308 controls the electrical power transferred between electrical powersource 302 and application load 306, between electrical power source 302and electrical energy storage unit 310, and between application load 306and electrical energy storage unit 310. Electrical power P₁ 331represents the output electrical power from electrical power source 302.Electrical power P₂ 333 represents the input electrical power drawn byapplication load 306, which, in this example, is a cyclic load.Electrical power P₃ 335 represents the electrical power generated byapplication load 306 in the regeneration region. Electrical power P₄ 337represents the electrical power received by electrical energy storageunit 310 from application load 306. Electrical power P₅ 339 representsthe output electrical power from electrical energy storage unit 310.

An example of electrical energy storage unit 310 is an ultracapacitor,which is characterized by high power densities. For increased electricalenergy storage, multiple ultracapacitors may be connected in series andparallel to form an ultracapacitor bank. Electrical current flowing intoan ultracapacitor charges the ultracapacitor, and electrical energy isstored via charge separation at an electrode-electrolyte interface. Thestored electrical energy may then later be used to output an electricalcurrent. In FIG. 3A, electrical power P₃ 335 generated by applicationload 306 may be fed as electrical power P₄ 337 to charge electricalenergy storage unit 310. In addition, electrical power P₁ 331 output byelectrical power source 302 may be fed as electrical power P₅ 339 tocharge electrical energy storage unit 310.

FIG. 3B shows plot 390 of the electrical power demand P₂ 333 (verticalaxis) of application load 306 as a function of time t (horizontal axis).Note that, in this example, the power ranges from positive to negativevalues. When the electrical power demand is positive (motoring region),application load 306 is drawing electrical power. When the electricalpower demand is negative (regeneration region), application load 306 isgenerating electrical power. As shown in plot 390, application load 306generates electrical power during time interval 367 (t₁≦t≦t₂), timeinterval 369 (t₃≦t≦t₄), and time interval 371 (t₅≦t≦t₆). Since energy isthe integral of power over time, area 366, area 368, and area 370represent the electrical energy generated by application load 306 duringtime interval 367, time interval 369, and time interval 371,respectively. This electrical energy is stored in electrical energystorage unit 310.

In an embodiment of the invention, electrical power drawn fromelectrical energy storage unit 310 is used to reduce peak electricalpower drawn from electrical power source 302. FIG. 3C shows the plot 392of the output electrical power P₁ 331 (vertical axis) from electricalpower source 302. In this example, the lower limit of P₁ 331 is zero. Asdiscussed in examples below, the lower limit may also be greater thanzero or less than zero, depending on the cycle and the storage capacityof electrical energy storage unit 310. When the output electrical poweris positive, electrical power is drawn from electrical power source 302.When the output electrical power is negative, electrical power is fedback into electrical power source 302. In the example shown, in themotoring region, an upper limit P_(UL) 394 is placed on the outputelectrical power P₁ 331. For values of P₂≦P_(UL) (plot 390 in FIG. 3B),P₂ is supplied only by P₁. For values of P₂>P_(UL), P₁ supplies a valueof P_(UL). The additional electrical power required is supplied by P₅339 drawn from electrical energy storage unit 310.

Referring to FIG. 3B, P₂ is greater than P_(UL) during time interval 361(T₁≦t≦T₂), time interval 363 (T₃≦t≦T₄), and time interval 365 (T₅≦t≦T₆).Note that the difference P₂−P_(UL) are represented by pulses, referencedas pulse 350, pulse 352, and pulse 354, respectively. The pulseamplitudes are referenced as amplitude 380, amplitude 382, and amplitude384, respectively. The energy drawn within each pulse is referenced aspulse energy 360, pulse energy 362, and pulse energy 364, respectively.As discussed above, energy is represented by area in a power vs. timeplot. In the example shown, the pulses have a triangular shape. Ingeneral, the pulse shape may vary, depending on the application load andoperating conditions and other factors.

In an embodiment of the invention, the electrical energy storage unit310 is configured such that it may supply all pulse energy requiredduring the operation of application load 306. Parameters to beconsidered in configuring the electrical energy storage unit 310 includepulse amplitude, pulse width, pulse shape, and time interval betweenpulses. If P₃ is not sufficient to maintain adequate charge inelectrical energy storage unit 310, P₁ may also be used during off-peakperiods to charge electrical energy storage unit 310.

In an embodiment of the invention, if application load 306 operates onlyin the motoring region (no regeneration), electrical energy storage unit310 may be charged entirely by P₁ 331 from electrical power source 302.The charging may occur during off-peak demand to limit the power P₁ 331from electrical power source 302 during peak demand, FIG. 3D shows plot3100 of the electrical power demand P₂ 333 (vertical axis) ofapplication load 306 as a function of time t (horizontal axis). In thisexample, P₂ ranges from zero to positive values. When the electricalpower demand is positive, application load 306 is drawing electricalpower.

FIG. 3E shows the plot 3200 of the output electrical power P₁ 331(vertical axis) from electrical power source 302. When the outputelectrical power is positive, electrical power is drawn from electricalpower source 302. In the example shown, an upper limit P_(UL) 3144 isplaced on the output electrical power P₁ 331. For values of P₂≦P_(UL)(plot 3100 in FIG. 3D), P₂ is supplied only by P₁. For values ofP₂>P_(UL), P₁ supplies a value of P_(UL). The additional electricalpower required is supplied by P₅ 339 drawn from electrical energystorage unit 310. In FIG. 3D, note that P₂>P_(UL) for pulse 3102-pulse3110. The corresponding pulse energies are pulse energy 3122-pulseenergy 3130, respectively, which are supplied by electrical energystorage unit 310. Note that a positive lower limit P_(LL) 3146 (FIG. 3E)may be placed on the output electrical power P₁ 331. Advantages ofmaintaining a lower limit are discussed below.

Electrical drive motors used in mining excavators typically operate on3-phase alternating current (AC). Mining excavators are typicallypowered from an electrical power distribution network feedinghigh-voltage AC power through high-voltage armored trail cables to theprimary side of a drive power transformer; more than one drive powertransformer may be used. A drive power transformer has multiplesecondary windings which supply power to a regenerative boost rectifiersystem through line reactors. Such a rectifier system may compriseactive front ends (AFEs). The active front ends are pulse-widthmodulated isolated gate bipolar transistor (IGBT) rectifiers thatconvert the incoming AC into direct current (DC) and store the energy inlow inductive DC link capacitors. A regenerative rectifier system mayalso be implemented with silicon-controlled rectifier (SCR) bridges. Thenumber of drive power transformers and the number of active front endsdepend on the total power requirement of the drive motors. Powerelectronic inverters invert the DC voltage available at the DC link toAC voltages that feed the motors.

Electrical energy regenerated by the motors may be fed back to thepublic utility electrical grid by the active front ends. Under a lineside fault condition, however, the active front end may not be able tosend back all the regenerated energy to the public utility electricalgrid, and the machine needs to shut down. The regenerated energy whichcould not be fed back to the public utility electrical grid may lead toan increase in the DC link voltage. Since high DC link voltages maydamage components, and also pose a safety hazard, protective circuitssuch as DC choppers and crowbars are added to the system to suppressexcessive DC link voltages during regeneration. In this instance,electrical energy is converted to waste heat. As discussed below, in anembodiment of the invention, regenerated electrical energy is stored inan ultracapacitor bank. In another embodiment, the regeneratedelectrical energy stored in the ultracapacitor bank is used to provideauxiliary power for various systems and devices such as computers,displays, control systems, lighting systems and others.

FIG. 4 shows a single-line diagram of electric shovel control system400. Block 401 represents input electrical power plant. Block 403represents electrical power converters. Block 405 represents electricaldrive motors.

Referring to block 405, electric shovel 100 is equipped with sixelectrical drive motors, referenced as motor 4100 motor 4110. Each motorruns on 3-phase alternating current (AC).

In the example shown in block 401, electrical power is directly fed froma public utility electrical power grid via substation 404 supplying3-phase AC power at a voltage of 3.3 or 6.6 kV. Substation 404 isconnected via switch 406 to current transducer 408 and switch 410. Poweris supplied via fuse 412 and switch 414 to the primary side of drivepower transformer 420. Similarly, power is supplied via fuse 422 andswitch 424 to the primary side of drive power transformer 426. Potentialtransducer 416 generates a synchronizing voltage feedback signal 418.

Referring to block 403, one output from the secondary side of drivepower transformer 420 is connected via current transducer 430 and activefront end (AFE) choke/reactor 438 to AFE AC-to-DC converter 446. Asecond output from the secondary side of drive power transformer 420 isconnected via current transducer 432 and AFE choke/reactor 440 to AFEAC-to-DC converter 448. Similarly, one output from the secondary side ofdrive power transformer 426 is connected via current transducer 434 andAFE choke/reactor 442 to AFE AC-to-DC converter 450. A second outputfrom the secondary side of drive power transformer 426 is connected viacurrent transducer 436 and AFE choke/reactor 444 to AFE AC-to-DCconverter 452. Output DC voltages are monitored by potential transducer454 and potential transducer 456.

DC power from the outputs of AFE AC-to-DC converter 446-AFE AC-to-DCconverter 452 is fed to the inputs of AFE DC-to-AC inverter 458-AFEDC-to-AC inverter 464. Block 480 represents a ground fault detectioncircuit. Block 482 represents an overvoltage chopper circuit thatdissipates excess electrical energy through resistor 484. The outputs ofAFE DC-to-AC inverter 458-AFE DC-to-AC inverter 464 are connectedthrough current transducer 466-current transducer 472, respectively, tomotor 4100—motor 4110. Switching drive power between different motorsmay be performed via transfer switch 490/492.

FIG. 5 shows a schematic of an ultracapacitor bank electrical energystorage unit integrated into an existing electrical power convertersystem. The existing electrical power converter system is represented bydrive power transformer 502, AFE choke/reactor 504, AFE choke/reactor506, AFE 508, AFE 510, DC link 512 and inverter 514. Motor 560represents a cyclic load. As previously shown in FIG. 4, inverter 514may feed more than one motor. The ultracapacitor electrical energystorage unit 540 comprises DC-to-DC converter 542/544, choke/reactor546, and ultracapacitor bank 548. The ultracapacitor electrical energystorage unit 540 may be disconnected from the electrical power convertersystem via disconnect switch 550. The ultracapacitor electrical energystorage unit 540 is managed by ultracapacitor energy managementcontroller 570.

Note that a mining excavator with a system of multiple electric motorsmay be viewed as a single unified cyclic load that operates duringmotoring intervals and regeneration intervals. The electric shovel 100in FIG. 1 appears as a cyclic load to the public utility electrical gridgoverned by a duty cycle. FIG. 6 shows an example of a power cycle forelectric shovel 100 over a 30-sec duty cycle. The horizontal axis 602represents time in seconds (sec). The vertical axis 604 represents powerin kilowatts (kW). Plot 606 represents the power demand of electricshovel 100.

In this example, note that on vertical axis 604, the power ranges frompositive values to negative values. For positive values, indicated bymotoring region 608, the electric shovel 100 is drawing power. Fornegative values, indicated by regeneration region 610, the electricshovel 100 is generating power. In the duty cycle shown in FIG. 6, thereare three time intervals during which electric shovel 100 operates inthe regeneration region 610: t₁ 641/−t₂ 643, t₃ 645−t₄ 647, and t₅649−t₆ 651.

The maximum power demand presented by electric shovel 100 is P_(max)612. In the standard electrical power converter system, all power issupplied by drive power transformer 502 (FIG. 5). Therefore, in motoringregion 608, plot 606 also represents the power supplied by drive powertransformer 502. In an embodiment of the invention, the maximum powersupplied by drive power transformer 502 is set at a user-defined valueP_(UL) 616 (UL=Upper Limit). In an embodiment of the invention, duringthe time intervals in which peak demand exceeds P_(UL) 616, the powerexceeding P_(UL) 616 is supplied by the ultracapacitor bank 546. Oneskilled in the art may set the value of P_(UL) 616 according to specificequipment and applications.

FIG. 7 shows a modified power profile of drive power transformer 502.The horizontal axis 602 represents the 30-sec duty cycle previouslyshown in FIG. 6. The vertical axis 704 represents power in kilowatts(kW). Plot 706 represents the power delivered by drive power transformer502. Note that the power falls between P_(UL) 616 and P_(LL) 718(LL=Lower Limit). The lower limit P_(LL) 718 may be set to zero, anegative value, or a positive value. The lower limit is set to zero ifdrive power transformer 502 delivers zero power during a regenerationinterval, and all regenerated electrical energy is stored inultracapacitor bank 546. The lower limit is set to a negative value ifthe capacity of ultracapacitor bank 546 is not sufficient to store allof the regenerated electrical energy: a portion of the regeneratedelectrical energy is stored in ultracapacitor bank 546, and a portion ofthe regenerated electrical energy is returned to the public utilityelectrical grid. The lower limit is set to a positive value (as shown inthe example in FIG. 7) if the regenerated electrical energy is notsufficient to fully charge ultracapacitor bank 546: electrical powerfrom drive power transformer 502 is also used to charge ultracapacitorbank 546 during off-peak intervals. Note that line power ripple isdecreased as P_(LL) 718 is increased. It is therefore advantageous toset P_(LL) 718 as high as possible, consistent with the duty cycle andvoltage of electrical energy storage unit 310. One skilled in the artmay set the value of P_(LL) 718 according to specific equipment andapplications.

FIG. 8 shows the corresponding ultracapacitor power profile. Thehorizontal axis 602 represents the 30-sec duty cycle previously shown inFIG. 6. The vertical axis 804 represents power in kilowatts (kW). Plot806 represents the power profile of ultracapacitor bank 546 (FIG. 5).Note that on vertical axis 804, the power ranges from positive values tonegative values. For positive values, indicated by regeneration region808, the ultracapacitor bank 546 is drawing power (charging theultracapacitor bank). For negative values, indicated by motoring region810, the ultracapacitor bank 546 is generating power (discharging theultracapacitor bank).

FIG. 9 shows the energy stored in the ultracapacitor system. Thehorizontal axis 602 represents the 30-sec duty cycle previously shown inFIG. 6. The vertical axis 904 represents energy in kilojoules (kJ). Plot906 is calculated by integrating the power (represented by plot 806 inFIG. 8) as a function of time. This data is used for proper sizing ofultracapacitor bank 546. Once the ultracapacitor bank 546 has therequired energy stored, additional energy, if available, could be sentback to the public utility electrical grid. As an example, the operatingvoltage of the ultracapacitor system is approximately 1400 to 1800volts, and the total capacitance of the ultracapacitor is approximately4.5 to 9 farads. One skilled in the art may set design requirements forthe ultracapacitor system according to specific equipment andapplications.

FIG. 10 is a flowchart summarizing steps for limiting the power drawnfrom an electrical power source by a cyclic load. In step 1002, theupper power limit P_(UL) and the lower power limit P_(LL) are set. Theprocess then passes to step 1004, in which the input power drawn by thecyclic load is measured. The process then passes to step 1006, in whichthe measured input power drawn by the cyclic load is compared to theupper and lower power limits of the electrical power source. If themeasured input power drawn by the cyclic load falls within the upper andlower power limits, then the process passes to step 1008, in whichnormal operation continues. If the measured input power drawn by thecyclic load does not fall within the upper and lower power limits, thenthe process passes to step 1010, in which the measured input power drawnby the cyclic load is compared to the upper power limit P_(UL) and thelower power limit P_(LL). If the measured input power drawn by thecyclic load is greater than the upper power limit P_(UL), the processthen passes to step 1014, in which the ultracapacitor bank suppliespower to the DC link. If the measured input power drawn by the cyclicload is less than the lower power limit P_(LL), the process then passesto step 1012, in which the ultracapacitor bank draws power from the DClink. As discussed above, the ultracapacitor bank may be charged fromthe electrical power source. If the cyclic load operates in both amotoring region and a regeneration region, the ultracapacitor bank mayalso be charged from electrical power generated by the cyclic load.

Upon completion of either step 1012 or step 1014, the process passes tostep 1016, in which the voltage across the ultracapacitor bank ismonitored. The process then returns to step 1002, in which the upperpower limit and the lower power limit are reset if needed.

In addition to having a very high power density, ultracapacitors areable to withstand a relatively large number of charge-discharge cyclesin comparison to conventional batteries. Ultracapacitors are also ratedfor operation at temperatures as low as 40° C. below zero and are thusmore suitable for use in mining operations which take place in severeweather conditions. In accordance with the present invention, theultracapacitor bank 548 is used to provide auxiliary power for varioussystems and devices such as computers, displays, control systems,lighting systems and others. The ultracapacitor bank 548 is configuredto provide a nominal operating voltage sufficient to enable operation ofa mining excavator. During use, the ultracapacitor bank 548 isdischarged and the voltage level decreases to a lower voltage limit atwhich the voltage level is not sufficient to maintain operation of themining excavator. In one embodiment, the ultracapacitor bank 548 isconfigured to provide a nominal voltage level of approximately 1800V. Inaddition, an exemplary lower voltage limit may be approximately 1500V.It has been determined that it takes approximately 8 hours of normaloperation of the mining excavator for the voltage level to decrease from1800V to 1500V. Therefore, in one embodiment ultracapacitor bank 548holds sufficient energy to provide 8 hours of backup power for variousmining excavator electronic systems and components such as computers,displays, control systems, GIS, lighting systems and others. Thiseliminates the need for conventional batteries as a source of backuppower, thus increasing reliability.

Referring to FIG. 12, a method for operating the ultracapacitor bank 548is shown. At step 1300, line power is measured by the potentialtransducer 416 described in relation to FIG. 4. If line power is presentat step 1320, normal operation of the mining excavator continues at step1340. If line power is not present at step 1320, the potentialtransducer 454 (see FIG. 4) then checks the voltage level of theultracapacitor bank 548 at step 1360. If the voltage level is above thevoltage lower limit at step 1380, then power is supplied to theultracapacitor energy management controller 570 at step 1400. If thevoltage level is below the voltage lower limit at step 1380, then themining excavator control system is shutdown at step 1420. After bothsteps 1400 and 1420, the voltage level of the ultracapacitor bank 548 ismonitored at step 1440 by the potential transducer 454. The line voltageis again checked at step 1300.

The maximum permissible power supplied by AFE 508 and AFE 510 (see FIG.5) in motoring and regeneration mode has to be considered in order tooptimize the peak power of the system. With respect to FIG. 11, themaximum motoring power to be supplied by the AFE 508 and AFE 510 isselected as PMAX=950 MW, while the maximum regenerated power that can befed back by the AFE to the grid is selected as PMIN=−200 MW. The extraregenerated power is used to charge the ultracapacitor bank 458 which israted at 1800V, for example. The DC-to-DC converter 542/544 serves toexchange power between DC link 512 and the ultracapacitor bank 458.Applying the above conditions, the power-time graph is presented in FIG.13 for the profile of FIG. 11. In particular, FIG. 13 depicts a powerdemand plot 1220, a line power plot 1240 which coincides with portionsof the power demand plot 1220, a ultracapacitor power plot 1260 andultracapacitor voltage plot 1280. From FIG. 13, the total regenerativeenergy that can be captured by the ultracapacitor bank 458 is determinedto be approximately 2 MJ by calculating the integral of power.

Referring to FIG. 14, Table 1 lists important items in a GIS whichrequire backup. It is determined that the energy requirement for GISbackup systems is approximately 2 MJ. Hence, the total energyrequirement for the ultracapacitor bank 458 is approximately 4 MJ. Fromsimulation the peak current requirement of the ultracapacitor bank 458is found to be 300 Amps. From these data the cost and size of theultracapacitor bank 458 may be estimated.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. R is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present invention and that variousmodifications may be implemented by those skilled in the art withoutdeparting from the scope and spirit of the invention. Those skilled inthe art could implement various other feature combinations withoutdeparting from the scope and spirit of the invention.

1. A method for operating an ultracapacitor system used in a miningexcavator powered by an electrical power source, comprising the stepsof: detecting whether a line power from the electrical power source ispresent; measuring a voltage level of the ultracapacitor system if theline power is not present; comparing the measured voltage level with aminimum voltage level for the ultracapacitor system; and supplyingauxiliary power from the ultracapacitor system to operate miningexcavator systems if the measured voltage level is more than the minimumvoltage level.
 2. The method according to claim 1 further including thestep of stopping operation of the mining excavator if the measuredvoltage is less than the minimum voltage level.
 3. The method accordingto claim 1 wherein the ultracapacitor system is configured to provideapproximately 8 hours of normal operation.
 4. The method according toclaim 1 wherein the minimum voltage level is 1500V.
 5. The methodaccording to claim 1 wherein a nominal voltage for the ultracapacitorsystem is approximately 1800V.
 6. The method of claim 1, furthercomprising the step of charging the ultracapacitor system withelectrical power drawn from the electrical power source.
 7. The methodof claim 1, wherein the electrical power source comprises a directconnection to a public utility electrical power grid.
 8. The method ofclaim 1, wherein the electrical power source outputs three-phasealternating current with a minimum voltage of at least one kilovolt. 9.The method of claim 8, wherein the minimum voltage is at least threekilovolts.
 10. The method of claim 1, wherein the at least one miningexcavator comprises at least one of: an electric shovel; and a dragline.11. A method for operating an ultracapacitor system used in a miningexcavator powered by an electrical power source, comprising the stepsof: operating the mining excavator to provide a cyclic load; chargingthe ultracapacitor system with electrical power generated by the cyclicload during a regeneration interval; detecting whether a line power fromthe electrical power source is interrupted; measuring a voltage level ofthe ultracapacitor system if the line power is interrupted; comparingthe measured voltage level with a minimum voltage level for theultracapacitor system; and supplying auxiliary power from theultracapacitor system to operate mining excavator systems if themeasured voltage level is more than the minimum voltage level.
 12. Themethod of claim 11, further comprising the step of charging theultracapacitor system with electrical power drawn from the electricalpower source.
 13. The method of claim 11, wherein the electrical powersource comprises a direct connection to a public utility electricalpower grid.
 14. The method of claim 11, wherein the electrical powersource outputs three-phase alternating current with a minimum voltage ofat least one kilovolt.
 15. The method of claim 14, wherein the minimumvoltage is at least three kilovolts.
 16. The method of claim 11, whereinthe at least one mining excavator comprises at least one of: an electricshovel; and a dragline.
 17. The method according to claim 11 furtherincluding the step of stopping operation of the mining excavator if themeasured voltage is less than the minimum voltage level.
 18. The methodaccording to claim 11 wherein the ultracapacitor system is configured toprovide approximately 8 hours of normal operation.
 19. The methodaccording to claim 11 wherein the minimum voltage level is 1500V. 20.The method according to claim 11 wherein a nominal voltage for theultracapacitor system is approximately 1800V.